1. Field of the Invention
The present invention relates to a magnetic head drive for a magnetic tape recorder of helical scan system.
2. Description of the Prior Art
In a video cassette recorder or the like by using helical scan, noises are generated when a magnetic tape is played back at a speed different from that on recording, for example for still reproduction or the like. The noises are generated when tracks of a reproduction head does not coincide with record tracks. In order to reduce such noises, various magnetic head drives for a magnetic head have been proposed for automatic tracking on reproduction. They are divided into two groups.
In one group, a magnetic head drive has a moving coil for magnetization mounted to a plate spring supporting a magnetic head. Therefore, it has problems that leakage magnetic flux is difficult to be prevented and that it is difficult to be produced compactly.
In another group to which the present invention relates, one or two bimorph elements each made of a pair of piezoelectric plates adhered to each other are used to drive the magnetic head so as to control its position. An applied voltage to the bimorph element expands one of the two plates while contracts the other, and the bimorph element is distorted according to the applied voltage. A drive circuit for controlling the bimorph element is disclosed for example in U.S. Pat. No. 4,169,276. It is proposed to use a pair of bimorph elements in order to make the magnetic head contact with a magnetic tape normally (for example, Japanese Utility Model laid open Publication 180,221/1984, Japanese Patent laid open Publication 173,922/1990 and Japanese Utility Model laid open Publication 58,810/1992). An end of each of the bimorph elements is fixed to a support member at one end thereof, while the other free ends thereof are connected to a bridge member with a magnetic head mounted thereto. The bridge member is made of a material such as glass fiber flexible for bending. In a drive using such a pair of bimorph elements, when a voltage is applied to the bimorph element, they are bent due to piezoelectric effect to move the magnetic head in the direction of track width. Because a replacement quantity of the magnetic head depends on the applied voltage, the applied voltage is controlled by detecting an envelope of reproduced signals.
However, such a prior art drive has a problem that a protrusion quantity above the outer surface of the drum decreases as the magnetic head is displaced. Especially, the magnetic head does not contact sufficiently with the magnetic tape in a region wherein a floating quantity of the magnetic tape is large.